Photoconductive elements containing organic photoconductors and sensitizers



United States Patent ABSTRACT OF THE DISCLOSURE Photoconductivecompositions and elements containing organic photoconductors and saltsof certain substituted cycloheptenyl compounds are described. The saltsare used a as sensitizers for the organic photoconductors.

This invention relates to electrophotography, and in particular to novelsensitized photoconductive compositions and elements having coatedthereon such compositions.

The process of xerography, as disclosed by Carlson in US. 2,297,691,employs an electrophotographic element comprising a support materialbearing a coating of a normally insulating material whose electricalresistance varies with the amount of incident actinic radiation itreceives during an imagewise exposure. The element, commonly termed aphotoconductive element, is first given a uniform surface charge,generally in the dark after a suitable period of dark adaptation. It isthen exposed to a pattern of actinic radiation which has the effect ofdifferentially reducing the potential of the surface charge inaccordance with the relative energy contained in various parts of theradiation pattern. The differential surface charge or electrostaticlatent image remaining on the electrophotographic element is then madevisible by contacting the surface with a suitable electroscopic markingmaterial. Such marking material or toner, whether contained in aninsulating liquid or on a dry carrier, can be deposited on the exposedsurface in accordance with either the charge pattern or in the absenceof charge pattern as desired. The deposited marking material may then beeither permanently fixed to the surface of the sensitive element byknown means such as heat, pressure, solvent vapor, or the like, ortransferred to a second element to which it may similarly be fixed.Likewise, the electrostatic latent image can be transferred to a secondelement and a developed there.

Various photoconductive insulating materials have been employed in themanufacture of electrophotographic elements. For example, vapors ofselenium and vapors of selenium alloys deposited on a suitable supportand particles of photoconductive zinc oxide held in a resinous,film-forming binder have found wide application in present-day documentcopying applications.

Since the introduction of electrophotography, a great many organiccompounds have also been screened for their photoconductive properties.As a result a very large number of organic compounds are known topossess some degree of photoconductivity. Many organic compounds haverevealed a useful level of photoconduction and have been incorporatedinto photoconductive compositions. Optically clear organicphotoconductor-containing elements having desirable electrophotographicproperties can be especially useful in electrophotography. Suchelectrophotographic elements may be exposed through -a trans parent baseif desired, thereby providing unusual flexibility in equipment design.Such compositions, when coated as a film or layer on a suitable supportalso yield an element 3,503,740 Patented Mar. 31, 1970 which isreusable; that is, it can be used to form subsequent images afterresidual toner from prior images has been removed by transfer and/orcleaning.

Although some of the organic photoconductors comprising the materialsdescribed are inherently light sensitive, their degree of sensitivity isusually low and in the short wave length portion of the spectrum so thatit is common practice to add materials to increase the speed and toshift the sensitivity toward the longer wave length portion of thevisible spectrum. Increasing the speed and shifting the sensitivity ofsuch systems into the visible regions of the spectrum has severaladvantages: it makes available inexpensive and convenient light sourcessuch as incandescent lamps; it reduces exposure time; it makes possiblethe recording of a wide range of colors in proper tonal relationship,and allows projection printing through various optical systems. Byincreasing the speed through the use of sensitizers, photoconductorswhich would otherwise have been unsatisfactory are useful in processeswhere high speeds are required such as document copying. However, whilesome of the sensitizers have appreciably increased the speed of aphotoconductive system, they have been unstable when exposed to moisturefor a period of time.

It is, therefore, an object of this invention to provide a novel classof sensitizers for use in combination with certain photoconductors sothat improved electrophotographic speeds are obtained.

Another object of this invention is to provide novel sensitizedphotoconductive elements.

It is also an object to provide novel sensitized photoconductivecompositions which can be positively and negatively charged.

It is a further object of this invention to provide novelphotoconductive compositions sensitized with a class of sensitizerswhich are stable when exposed to moisture for an extended period oftime.

These and other objects of this invention are accomplished withphotoconductive compositions containing a photoconductor and asensitizer which is a salt of a cycloheptenyl compound. The termcycloheptenyl compound is meant to include derivatives of cycloheptane,cycloheptene, cycloheptadiene and cycloheptatriene.

The cycloheptenyl salts of this invention can be sub stituted in thenucleus, if desired, with a wide variety of substituents such as:

(a) An aryl radical including substituted and unsubstituted arylradicals such as phenyl and naphthyl, and particularly phenyl radicalscontaining amino groups such as alkylamino groups, as well as arylradicals containing a fused heterocyclic ring,

(b) A heterocyclic radical including substituted or unsubstitutedheterocyclic radicals, and

(c) A hydrogen atom.

The salt is preferably a perchlorate or fiuoroborate but any anioncapable of forming a salt with the cycloheptenyl radical is suitable,e.g., sulfate, chloride, bromide, iodide,

etc.

When the cycloheptenyl salts of this invention are used in conjunctionwith composition containing conventional photoconductors, substantialincreases are noted in the electrical speeds of the composition. Also,these sensitizers are markedly stable to moisture when so exposed for aperiod of time.

The preferred cycloheptenyl salt sensitizers of this irivention arerepresented by the following structures:

. 3 where E, G and D can be either (a) A phenyl radical,

(b) A naphthyl radical,

(c) A heterocyclic radical having to 6 atoms in the heterocyclic nucleusand at least one hetero nitrogen atom, or

(d) A hydrogen atom;

(R and R (R and R (R and R and (R and R are together the necessary atomsto complete a benzene ring fused to the cycloheptenyl nucleus; X is ananion such as a perchlorate, fluoroborate, sulfate, chloride, bromide oriodide.

Typical cycloheptenyl compounds which belong to the herein'describedgeneral class of compounds include the following:

(1) trop ylium perchlonate,

(2) (4-dimethylaminophenyl)tropylium fluoroborate,

(3) 5-(4-dimethylaminophenyl)-5H-dibenzo[a,d]

tropylium fiuoroborate,

(4) 5-(Z-nrethyl-4-diethylaminophenyl)-5H-dibenzo [a,d]tropyliumfluoroborate,

(5) 5-(4-dimethylaminol-naphthyD-SH-dibenzoa,d]

tropylium fluoroborate,

(6) 5-(9-julolidyl)-5H-dibenzo[a,d]tropyliu'm fiuoroborate,

(7) 5H-dibenzo[a,d] tropylium fiuoroborate, and

(8) (5H-dibenzo[a,d]cycloheptenyl)-1,2,3-triazolyl fluoroborate.

Electrophotographic elements of the invention can be prepared with anyphotoconductive compound and the sensitizers of this invention in theusual manner, i.e., by blending a dispersion or solution of thephotoconductive compound together with a binder, when necessary ordesirable, and coating or forming a self-supporting layer with thephotoconductive composition. Generally, a suitable amount of thesensitizing compound is mixed with the photoconductive coatingcomposition so that, after thorough mixing, the sensitizing compound isuniformly distributed throughout the desired layer of the coatedelement. The amount of sensitizer that can be added to aphotoconductor-incorporating layer to give effective increases in speedcan vary widely. The optimum concentration in any given case will varywith the specific photoconductor and sensitizing compound used. Ingeneral, sub stantial speed gains can be obtained where an appropriatesensitizer is added in a concentration range from about 0.0001 to about30 percent by weight based on the weight of the film-forming coatingcomposition. Normally, a sensitizer is added to the coating compositionin an amount from about 0.005 to about 5.0 percentby weight of the totalcoating composition.

The sensitizers of this invention are effective for enhancing theelectrophotosensitivity of a wide variety of photoconductors. Thepreferred photoconductors are those organic compounds which exhibit anelectrophotosensitivity to light and are capable of forming transparentelements. An especially useful class of organic photoconductors isreferred to herein as organicamine photoconductors. Such organicphotoconductors have as a common structural feature at least one aminogroup. Useful organic photoconductors which can be spectrally sensitizedin accordance with this invention include, therefore, arylaminecompounds comprising (1) diarylamines such as diphenylamine,dinaphthylamine, N ,N'-diphenylbenzidine,

,N phenyl 1 naphthylamine; N phenyl 2 naphthylamine; N,N diphenyl pphenylenediamine; 2 carboxy 5 chloro 4' methoxydiphenylamine; panilinophenol; N,N' di 2 naphthyl p phenylenediamine;

4,4 benzylidene bis N,N diethyl m toluidine), those described in Fox US.Patent 3,240,597 issued Mar. 15, 1966, and the like, and (2)triarylamines including (a) nonpolymeric triarylamines, such astriphenylamine, N,N,N',N' tetraphenyl m phenylenediamine; 4acetyltriphenylamine, 4 hexanoyltriphenylamine; 4-lauroyltriphenylamine; 4 hexyltriphenylamine, 4 dodecyltriphenylamine,4,4 bis(diphenylamino)benzyl, 4,4- bis(diphenylamino)benzophenone, andthe like, and (b) polymeric triarylamines such aspoly[N,4"-(-N,N',N-triphenylbenzidinefl; polyadipyltriphenylamine,polysebacyltriphenylamine; polydecamethylenetriphenylamine; poly- N (4vinylphenyl)diphenylamine, poly N (vinylphenyl) 00,12 dinaphthylamineand the like. Other useful amine-type photoconductors are disclosed inUS. Patent 3,180,730 issued Apr. 27, 1965.

Useful photoconductive substances capable of being spectrally sensitizedin accordance with this invention are disclosed in Fox U.S. Patent3,265,496 issued Aug. 9, 1966, and include those represented by thefollowing general formula:

GI:NA] Q, A

wherein A represents a mononuclear or polynuclear divalent aromaticradical, either fused or linear (e.g., phenyl, naphthyl, biphenyl,binaphthyl, etc.), or a substituted divalent aromatic radical of thesetypeswherein said substituent can comprise a member such as an acylgroup having from 1 to about 6 carbon atoms (e.g., acetyl, propionyl,butyryl, etc.), an alkyl group having from 1 to about 6 carbon atoms(e.g., methyl, ethyl, propyl, butyl, etc.), an alkoxy group having from1 to about 6 carbon atoms (e.g., methoxy, ethoxy, propoxy, pentoxy,etc.), or a nitro group; A represents a mononuclear or polynuclearmonovalent aromatic radical, either fused or linear (e.g., phenyl,naphthyl, biphenyl, etc.); or a substituted monovalent aromatic radicalwherein said substituent can comprise a member, such as an acyl grouphaving from 1 to about 6 carbon atoms (e.g., acetyl, propionyl, butyryl,etc.), an alkyl group having from 1 to about 6 carbon atoms (e.g.,methyl, ethyl, propyl, butyl, etc.), an alkoxy group having from 1 toabout 6 carbon atoms (e.g., rnethoxy, propoxy, pentoxy, etc.), or anitro group; Q can represent a hydrogen atom, a halogen atom or anaromatic amino group, such as ANH; b represents an integer from 1 toabout 12, and G represents a hydrogen atom, a mononuclear or polynucleararomatic radical, either fused or linear (e.g., phenyl, naphthyl,biphenyl, etc.), a substituted aromatic radical wherein said substituentcomprises an alkyl group, an alkoxy group, an acyl group, or a nitrogroup, or a poly-(4'-vinylphenyl) group which is bonded to the nitrogenatom by a carbon atom of the phenyl group.

Polyarylalkane photoconductors are particularly useful in producing thepresent invention. Such photoconductors are described in US. Patent3,274,000; French Patent 1,383,461 and in copending application of Seusand Goldman titled Photoconductive Elements Containing OrganicPhotoconductors, filed Apr. 3, 1967. These photoconductors include leucobases of diaryl or triaryl methane dye salts, 1,1,1-triarylalkaneswherein the alkane moiety has at least two carbon atoms andtetraarylmethanes, there being substituted an amine group on at leastone of the aryl groups attached to the alkane and methane moieties ofthe latter two classes of photoconductors which are non-leuco basematerials.

Preferred polyaryl alkane photoconductors can be rep resented by theformula:

wherein each of D, E and G is an aryl group and I is a hydrogen atom, analkyl group, or an aryl group, at least one of D, E and G containing anamino substituent. The aryl groups attached to the central carbon atomare preferably phenyl groups, although naphthyl groups can also be used.Such aryl groups can contain such substituents as alkyl and alkoxytypically having 1 to 8 carbon atoms, hydroxy, halogen, etc., in theortho, meta or para positions, ortho-substituted phenyl being preferred.The aryl groups can also be joined together or cyclized to form afluorene moiety, for example. The amino substituent can be representedby the formula wherein each L can be an alkyl group typically having 1to 8 carbon atoms, a hydrogen atom, an aryl group, or to gether thenecessary atoms to form a heterocyclic amino group typically having 5 to6 atoms in the ring such as morpholino, pyridyl, pyrryl, etc. At leastone of D, E, and G is preferably p-dialkylaminophenyl group. When I isan alkyl group, such an alkyl group more generally has 1 to 7 carbonatoms.

Representative useful polyarylalkane photoconductors include thecompounds listed below:

TABLE A Compound No.2

(1) 4,4 benzylidine bis(N,N diethylm-toluidine) (2) 4',4" diamino 4dimethylamino- 2,2"-dimethyltriphenylmethane.

-(3) 4',4" bis(diethylamino) 2,6 dichloro 2',2"dimethyltriphenylmethane.

(4) 4',4" bis(diethylamino) 2,2" dimethyldiphenylnaphthylmethane.

(5) 2,2" dimethyl 4,4,4 tris(dimethylamino triphenylmethane.

(6) 4',4" bis(diethylamino) 4 dimethylamino 2,2dimethyltriphenylmethane.

(7) 4',4" bis (diethylamino) 2 chloro- 2',2" dimethyl 4dimethylaminotriphenylmethane.

(8) 4',4 bis(diethylamino) 4 dimethylamino 2,2',2"trimethyltriphenylmethane.

(9) 4,4'-' bis(dimethylamino) 2 chlo- 4 I0 2,2"dimethyltriphenylmethane.

'(10) 4',4" bis(dimethylarnino) 2,2"-

, dimethyl 4 methoxytriphenylmethane.

(ll) Bis(4 diethylamino) 1,1,1 triphenylethane.

(12) Bis(4 diethylamino)tetraphenylmethane.

(13) 4,4 bis(benzylethylamino) 2',

2"-dimethyltriphenylmethane.

(14) 4,4" bis(diethylamino) 2,2" diethoxytriphenylmethane.

(15) 4,4 -bis(dimethylamino) 1,1,1 triphenylethane.

(l6) 1 (4 N,N dimethylaminophenyl) 1,1-diphenylethane. ('17)4-dimethylaminotetraphenylmethane. (18)4-diethylaminotetraphenylmethane.

Preferred binders for use in preparing the present photoconductivelayers comprise polymers having fairly high dielectric strength whichare good electrically insulating film-forming vehicles. Materials ofthis type comprise styrene-butadiene copolymers; silicone resins;styrene-alkyd resins; silicone-alkyd resins; soya-alkyd resins;"poly(vinyl chloride); poly(vinylidene chloride); vinylidene chloride. Lacrylonitrile copolymers; poly(vinyl acetate); vinyl acetate vinylchloride copolymers; poly(vinyl acetals), such as poly(vinyl butyral);polyacrylic and methacrylic esters, such as poly(methylmethacrylate),poly(n butylmethacrylate), poly(isobutyl methacrylate),

etc.; polystyrene; nitrated polystyrene; polymethylstyrene;

isobutylene polymers; polyesters, such aspoly(ethylenealkaryloxyalkylene terephthalate); phenol formaldehyderesins; ketone resins; polyamide; polycarbonates; polythiocarbonates;poly (ethyleneglycolco bishydroxyethoxyphenyl propane terephthalate);etc. Methods of making resins of this type have been described in theprior art, for example, styrene-alkyd resins can be prepared accordingto the method described in US. Patents 2,361,019 and 2,258,423. Suitableresins of the type contemplated for use in the photoconductive layers ofthe invention are sold under such trade names as Vitel PE-101, Cymac,Piccopale 100, Saran F-220 and Lexan 105. Other types of binders whichcan be used in the photoconductive layers of the invention include suchmaterials as paraffin, mineral Waxes, etc.

Solvents of choice for preparing coating compositions of the presentinvention can include a number of solvents such as benzene, toluene,acetone, Z-butanone, chlorinated hydrocarbons, e.g., methylene chloride,ethylene chloride, etc., ethers, e.g., tetrahydrofuran, or mixtures ofthese solvents, etc.

In preparing the coating composition useful results are obtained Wherethe photoconductor substance is present in an amount equal to at leastabout 1 weight percent of the coating composition. The upper limit inthe amount of photoconductor substance present can be widely varied inaccordance with usual practice. In those cases where a binder isemployed, it is normally required that the photoconductor substance bepresent in an amount from about 1 weight percent of the coatingcomposition to about 99 weight percent of the coating composition. Apreferred weight range for the photoconductor substance in the coatingcomposition is from about 10 weight percent to about 60 weight percent.

Coating thickness of the photoconductive composition on a support canvary widely. Normally, a coating in the range of about 0.001 inch toabout 0.01 inch before drying is useful for the practice of thisinvention. The preferred range of coating thickness was found to be inthe range from about 0.002 inch to about 0.006 inch before dryingalthough useful results can be obtained outside of this range.

Suitable supporting materials for coating the photoconductive layers ofthe present invention can include any of a wide variety of electricallyconducting supports, for example, paper (at a relative humidity above 20percent); aluminum-paper laminates; metal foils such as aluminum foil,zinc foil, etc.; metal plates, such as aluminum, copper, zinc, brass,and galvanized plates; vapor deposited metal layers such as silver oraluminum and the like. An especially useful conducting support can beprepared by coating a support material such as polyethyleneterephthalate with a layer containing a semiconductor dispersed in aresin. Such conducting layers both with and without insulating barrierlayers are described in US. Patent 3,245,833. Likewise, a suitableconducting coating can be prepared from the sodium salt of acarboXyester lactone of maleic anhydride and a vinyl acetate polymer.Such kinds of conducting layers and methods for their optimumpreparation and use are disclosed in U.S. 3,007,901 and 3,267,807.

The elements of the present invention can be employed in any of thewell-known electrophotographic processes which require photoconductivelayers. One such process is the aforementioned xerographic process. Asexplained previously, in a process of this type the electrophotographicelement is given a blanket electrostatic charge by placing the sameunder a corona discharge which serves to give a uniform charge to thesurface of the photoconductive layer. This charge is retained by thelayer owing to the substantial insulating property of the layer, i.e.,the low conductivity of the layer in the dark. The electrostatic chargeformed on the surface of the photo-conducting layer is then selectivelydissipated from the surface of the layer by exposure to light through an7 image-bearing transparency by a conventional exposure operation suchas, for example, by contact-printing technique, or by lens projection ofan image, etc., to form a latent image in the photoconducting layer. Byexposure of the surface in this manner, a charged pattern is created byvirtue of the fact that light causes the charge to be conducted away inproportion to the intensity of the illumination in a particular area.The charge pattern remaining after exposure is then developed, i.e.,rendered visible, by treatment with a medium comprisingelectrostatically attractable particles having optical density. The

developing electrostatically attractable particles can be in the form ofa dust, e.g., powder, a pigment in a resinous carrier, i.e., toner, or aliquid developer may be used in which the developing particles arecarried in an electrically insulating liquid carrier. Methods ofdevelopment of this type are widely known and have been described in thepatent literature in such patents, for example, a U.S. 2,297,691, and inAustralian Patent 212,- 315. In processes of electrophotographicreproduction such as in xerography, by selecting a developing particlewhich has as one of its components, a low-melting resin, it is possibleto treat the developed photoconductive material with heat and cause thepowder to adhere permanently to the surface of the photo-conductivelayer. In other cases, a transfer of the image formed on thephotoconductive layer can be made to a second support, which would thenbecome the final print. Techniques of the type indicated are well knownin the art and have been described in a number of US. and foreignpatents, such as US. Patents 2,297,691 and 2,551,582, and in RCA Review,vol. 15 1964), pages 469-484.

The present invention is not limited to any particular mode of use ofthe new electrophotographic materials,

and the exposure technique, the charging method, the transfer (if any),the developing method, and the fixing method as well as the materialsused in these methods can be selected and adapted to the requirements ofany particular technique.

Electrophotographic materials according to the present invention can beapplied to reproduction techniques wherein difierent kinds ofradiations, i.e., electromagnetic radiations as well as nuclearradiations, can be used. For this reason, it is pointed out herein thatalthough materials according to the invention are mainly intended foruse in connection with methods comprising an exposure, the termelectrophotography wherever appearing in the description and the claims,is to be interpreted broadly and understood to comprise both xerographyand xeroradiography.

The invention is further illustrated by the following examples whichinclude preferred embodiments thereof.

EXAMPLE 1 Preparation of tropylium perchlorate To a solution of 6.86 g.(0.02 m.) of triphenylmethyl perchlorate in about 150 ml. ofacetonitrile is added 1.84 g. 0.02 m.) of cycloheptatriene. The solventis removed and the residue triturated with ether. The solid is collectedon a filter and recrystallized from acetonitrileethylacetate. The yieldis 92% (3.5 g.) and the melting point is greater than 280 C.

EXAMPLE 2 Preparation of N,N-dimethylaminophenyl tropylium fluoroborateThis compound is prepared by the method of Jutz and Voithenleitner, Ber.Dtsch. Chem. Ges. 97:29 (1964) p. 45

8 EXAMPLE 3 Preparation of 5-(4-dimethylaminophenyl)-5H I dibenzo [a,d]tropylium fluoroborate A solution of 0.1 mole of N,N-dimethylaniline in50 ml. of 1,2-dimethoxyethane is added to a solution (0.048 mole) of 5,5dichloro-SI-I-di-benzo[a,d]cycloheptene in 50 ml. of1,2-dimethoxyethane. After one hour the solvent is removed and theresidue dissolved in 200 ml. of 3 percent HCl. An excess of fluoroboricacid is added and the oil that separates is taken up in ml. of pro;pionic anhydride. The product is precipitated by the addition of etherand recrystallized from ethyl acetate-nitromethane. The yield is 8.0 g.(42 percent) of a solid which exhibits a melting point of 242-243 C.

EXAMPLE 4 Preparation of 5-(4-dimethylamino-1-naphthyl)-5H- dibenzo[a,d]tropylium fluoroborate The precedure of Example 3 is followed, withthe exception that 0.1 mole of 4(N,N-dimethyl)-1-naphthalene was used inplace of the N,N-dimethylaniline. The product is obtained in 44 percentyield and has a melting point of 283-285 C. after purification byrecrystallization from acetonitrile.

EXAMPLE 5 Preparation of 5-(Z-methyl-4-diethylaminophenyl)- SH-dibenzo[a,d]tropylium fluoroborate The procedure of Example 3 is followed,using as the starting material 0.1 mole of 3-methyl-N,N-(diethylanilinein place of the N,N-dimethylaniline. The product is obtained in 14percent yield and has a melting point of 233-235 C. afterrecrystallization from acetonitrileethyl acetate mixture.

EXAMPLE 6 Preparation of 5-(9-julolidy1)-5H-dibenzo [a,d]tropyliumfluoroborate The procedure of Example 3 is followed, using as thestarting material 0.1 mole of julolidine in place of theN,N-dimethylaniline. The product is obtained in 24 percent yield and hasa melting point of 214215 C. after recrystallization from acetonitrile.

EXAMPLE 7 Photoconductive compositions containing cycloheptenyl saltsensitizers of the type described herein are separately incorporatedinto a coating dope having the following composition:

Organic photoconductor-0.15 g.

Polymeric binder-0.50 g.

Sensitizer0.002 g.

Methylene chloride or tetrahydrofuran5 ml.

These compositions are then separately coated on a wet thickness of0.004 inch on an aluminum surface maintained at 100 F. to provide thecoatings described in Table 1 below. In a darkened room, the surface ofeach of the photoconductive layers so prepared is either positively ornegatively charged to a potential of about 600 volts under a coronacharger. The charged layer is exposed through a stepped density grayscale to the radiation from a 60-watt incandescent lamp at a distance of100 cm. which provides an illumination-intensity of about 75meter-candles. The exposure causes reduction of the surface potential ofthe element under each step of the gray scale from its initialpotential, V to some lower potential, V, whose exact value depends uponthe actual amount of exposure received by the area. The element isexposed through each step ofthe scale and the resultant surfacepotential measured. The results of these measurements are plotted on agraph of surface potential V versus log of the exposure for each step.The actual speed of each element is expressed in terms of the reciprocalof the exposure required to reduce the surface potential to any fixedarbitrarily assigned value. The results of these measurements are setforth in the following Table 1. The speed is the quotient of 10 dividedby the exposure in meter-candle-seconds required to reduce the potentialby 100 volts. The polymeric binder used in the coating compositions is apolyester of terephthalic acid and a mixture of ethylene glycol (1 partby weight) and 2,2-bis(4- hydroxy-ethoxyphenyl) propane (9 parts byweight). The photoconductors referred to in the table are as follows:

MTriphenylamine N1,3,S-triphenyl-Z-pyrazoline O4,4'-bis (diethylamino)-2,2-dimethyltriphenylmethane P2,3,4,5-tetraphenylpyrroleQ-4,4'-bis(diethylamino)benzophenone TABLE 1 Speed Photosensitizerconductor 5-(4-dimethylaminophenyl)-5H-dibenzo M 400 320 [a,d]tropyliumfluoroborate. N 100 100 O 160 100 P 80 120 Q 120 63 5-(9-1ulolidyl)-5H-d1benzo[a,d]tropylium M 160 100 fluoroborate. N 250 100 20080 P 250 100 Q, 120 32 -(4-dimethylamino-l-naphthyl)-5H- M 320 50dibenzo[a,d]tropylium fiuoroborate. N 160 25 O 63 16 P 120 Q 164 12 5-(2methyl 4-diethylaminophenyl)5H- M 200 63 d1benzo[a,d]tropyliumfluoroborate. N 320 80 O 200 50 P 200 50 Q, 160 4-(d1methylaminophenyl)-tropylium N fluoroborate 0 30 Tropylium perchlorate M None M 16 EXAMPLE8 Coating compositions containing the sensitizers of this invention areprepared and coated in the manner described in Example 1. In a darkenedroom, the surface of each of the photoconductive layers so prepared ischarged to a potential of about +600 volts under a corona charger. Thelayer is then covered with a transparent sheet hearing a pattern ofopaque and light transmitting areas and exposed to the radiation from anincandescent lamp with an illumination intensity of about 75meter-candles for 12 seconds. The resulting electrostatic latent imageis developed in the usual manner by cascading over the surface of thelayer a mixture of negatively charged black thermoplastic tonerparticles and glass beads. A good reproduction of the pattern results ineach instance.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention as described hereinbefore and as defined in the appendedclaims.

We claim:

1. An electrophotographic element comprising a support having coatedthereon a photoconductive composition comprising an organicphotoconductor dispersed in a resin binder and a sensitizer having aformula selected from the group consisting of:

(a) a phenyl radical,

(b) a naphthyl radical,

(c) a heterocyclic radical having 5 to 6 atoms in the heterocyclicnucleus, and including at least one hetero nitrogen atom, and v (d) ahydrogen atom;

(R and R (R and R (R and R and (R and R are together the necessary atomsto complete a benzene ring fused to the cycloheptenyl nucleus; and

X is an anion.

2. The electrophotographic element of claim 1 wherein the sensitizer isa perchlorate salt of the cycloheptenyl compound.

3. The electrophotographic element of claim 1 wherein the sensitizer isa fluoroborate salt of the cycloheptenyl compound.

4. A photoconductive composition comprising an organic photoconductordispersed in a resin binder and a sensitizer selected from the groupconsisting of tropylium perchlorate, (4-dimethy1aminophenyl)-tropyliurnfluoro- 0 borate, 5-(4-dimethylaminophenyl) 5H dibenzo[a,d]

tropylium fluoroborate, 5-(2-methyl 4 diethylaminophenyD-SH-dibenzo[a,d]tropylium fiuoroborate, 5- (4-dimethylamino1-naphthyl) 5Hdibenzo[a,d]tropylium fluoroborate and 5-(9-julolidyl) 5Hdibenzo[a,d]tropylium fluoroborate.

5. An electrophotographic element comprising a support having coatedthereon the photoconductive composition of claim 4.

6. The electrophotographic element of claim 5 wherein the photoconductoris selected from the group consisting of triphenylamine, 1,3,5-triphenyl2 pyrazoline, bis(4- diethylamino)-l,l,1-triphenylethane, 4,4bis(diethy1- amino)-2,2'-dimethyltriphenylmethane, bis(4diethylamino)tetraphenylmethane, 2,3,4,5 tetraphenylpyrrole and4,4'-bis(diethylamino)-benzophenone.

7. A photoconductive element for use in electrophotography comprising asupport having coated thereon a photoconductive Composition comprising:

(a) about 10 to by weight based on said photoconductive composition ofan organic photoconductor,

(b) a film-forming polymeric binder for the said photoconductor, and

(c) 0.005 to 5% of 5-(4-dimethylaminophenyl) -5H- dibenzo[a,d]tropyliumfiuoroborate as a sensitizer for the said photoconductor.

8. A photoconductive element for use in electrophotography comprising asupport having coated thereon a photoconductive composition comprising:

(a) about 10 to 60% by weight based on said photoconductive compositionof an organic photoconductor,

(b) a film-forming polymeric binder for the said photoconductor, and

(c) 0.005 to 5% of 5-(9-julolidyl)-5H-dibenzo[a,d] tropyliumfluoroborate as a sensitizer for the said photoconductor.

9. A photoconductive element for use in electrophotography comprising asupport having coated thereon a photoconductive composition comprising:

(a) about 10 to 60% by weight based on said photoconductive compositionof an organic photoconductor,

(b) a film-forming polymeric binder for the said photo conductor, and

(c) 0.005 to 5% of 5-(4-dimethylamino-1-naphthyl)-5H-dibenzo[a,d]tropylium fluoroborate as a sensitizer for the saidphotoconductor.

10. A photoconductive element for use in electrophotography comprising asupport having coated thereon a photoconductive composition comprising:

(a) about 10 to 60% by weight based on said photoconductiv compositionof an organic photoconductor,

11. n w o n ia mm ng po ymeriebinder for the said ,photoconductor; and

- (c) 0.005 to 5%vof5-(2-methyl-4-diethyleminophenyl)-5H-dibenzo[a,'d]tropy1inm fluorobor'ate as a sensitizer for the saidphotoconductor. 1 11. 'A photoconductive element for useine'lectrophotogfra'phy comprising a support having coated thereon aph'oto'conductive' composition comprising: I (a) about 10 to 60% byweight based on'said photoconductive composition of an organic photocon-'d'ucto'r, (b) a film-forming polymeric binder for the saidphotoconductor, and 1 12 (c) 0.005 to'5% of' tropylium' perchlorate 'as'asensi tizer for the said photoconductor. t I References Cited" v NITED STATE PATENTS 3,141,770 7/1964 Davis et'a1.v4 9641' .'3,143,544 h8/1964 Van Dormail j. 260 24 O GEORGE F. LESME S, Primary Examiner 10J'. c, COOPER, As sist ant Exeminer U.S. Cl. X.R. 96-1.6

